It's almost impossible to forecast your future needs, that's why the $50k quote. They will install everything you could possibly ever need.

Wireless is possible, but at the very least install cat6 wire to some wireless ethernet access points throughout your house. That will ensure you have a strong signal no matter where you are. The stronger the signal, the greater the throughput. I use Ubiquiti Unifi access points in my house. For music around the house, check out the Sonos system, although I have never owned one myself. I use a Chromecast hooked to a TV, hooked to stereo gear, for music and video around my house. It's cheap and flexible. For the basement, simply wire the room for 5.1 sound or 10.2 sound or whatever you want.

For cameras, monitors, and other critical uses, I would use wired ethernet. I would not trust wifi for stuff like that. Just pre-determine their locations and run some cat6 to each location and back to a central switch. On an average size house, all this could be done for around $2000-3000 in materials and whatever you want to spend on labor, but you need to spend some time planning this first.

Use wired solutions first, then wireless where necessary. Wireless may sound like the easy fix but it can be problematic at times, I know I've pulled many hairs out trying to resolve wireless problems. I don't know what Smart Home solution was in the bid, but Control4 offers more reasonably priced solutions than what you where quoted. Problem with WiFi is that all but one brand of wireless access points is half duplex like a walkie talkie (send then receive) and each device has to be serviced by the wireless access point one at a time, wired networking is full duplex like a telephone (talk and listen at the same time) and the network switch can communicate to all devices simultaneously.